Chemical Reactions
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A chemical reaction involves a movement of valence electrons (i.e. electrons on the outside of an atom) from one atom to another or from one molecule to another. This happens by breaking a covalent bond on one molecule and generating a covalent bond on the other. Chemical reactions essentially causes atoms to be rearranged into new combinations. In contrast, a nuclear reaction involves the nucleus of the atoms rather than the valence electrons. The atoms themselves are changed into new atoms -- i.e. the atoms are not rearranged into new combinations, as in chemical reactions. For example, a neutron can split into a proton and an electron (an electron is also called a "beta" particle). Nuclear reactions also release enormous amounts of energy compared to chemical reactions. Energy is measured in Joules (J) or kilo-Joules (kJ or 10^3 J) or Mega-Joules (MJ or 10^6 J). Chemical reactions usually involve energy on the order of kilo-joules to Mega-Joules. Nuclear Reactions involve energy on the order of 10^12 Joules to 10^18 Joules (that difference is on the order of about a Thousand-Billion to a Million-Trillion Joules!). Isotopes are simply atoms of the same element that have a different mass due to a different number of protons or neutrons in the nucleus. There are two types of isotopes: radioactive (or unstable) and non-radioactive (or stable). For example, non-radioactive carbon has a molecular weight of 12. The radioactive isotope of carbon has a molecular mass of 14. A Hydrogen nucleus is, quite simply, a proton. An isotope of Hydrogen can have a nucleus made up of one proton and one neutron (therefore it is called Deuterium), or even one proton and two neutrons (and so it is called Tritium). Isotopic hydrogen atoms (deuterium or tritium) are not radioactive, but simply heavier (i.e. they are stable isotopes). Isotopes are often used in Biology for labelling specific molecules in order to follow or track them in a cell. Isotopes are also used in dating fossils. Carbon-14 (C-14) is sometimes used to label specific molecules. C-14 has a rather long half-life of 5600 years. A half-life is simply the length of time it takes half of all the unstable isotopes in a sample to decay. Other isotopes such as Sulfur-35 (S-35) and Phosphorus-32 (P-32) have half-lives of 87 days and 15 days respectively. P-32 is highly radioactive (much more radioactive than C-14), so more stringent safety precautions are necessary when using it in experiments. Another isotope, Iodine-125 (I-125) with a half-life of 60 days, is sometimes used to label specific proteins, such as antibodies, for in vitro experiments. Stable isotopes, such as deuterium, Oxygen-18, and Nitrogen-15, are used in other types of labelling experiments. Why do chemicals react? They do so to rearrange themselves into a lower energetic state. In the case of nuclear reactions, unstable atoms decay into a more stable state. So, if chemicals are always rearranging into the least energetic state, wouldn't it make sense that eventually there would be no more chemical reactions? Ultimately, the Universe itself may run down or die off in a "heat death" (i.e. all energy in the Universe will eventually be converted into heat). I would say that there is a possibility that, at some point in time, there may be no more "free-energy" (or usable energy) to drive chemical reactions. For the time being, however, we don't need to concern ourselves with this problem, as it won't happen for many billions of years. Instead let us focus on what is happening right now, here on Earth. First, chemical reactions may be driven if there is an input of energy. On Earth there are several potential sources of energy. Solar energy is the commonly known energy source. In particular, UV light is energetic enough to drive certain chemical reactions. There is also electric discharge, such as lightning, which is known to fix atmospheric Nitrogen gas (N2) into nitrates. And as already mentioned, man-made nuclear reactions release enormous amounts of energy: natural nuclear reactors (e.g. the Oklo reactor, which uses uranium-235 as fuel) may also generate enough energy to drive chemical reactions. I will deal with the implications of these energy sources later on in an "origins of life" section. In summary, life-forms use chemical reactions to grow and survive (however, life as we know it wouldn't be here without both chemical and nuclear reactions). On the practical side of things, scientists can use the properties of isotopes in experiments to help in understanding how living cells carry out their metabolic processes.
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